Magnetic resonance imaging (MRI) is generally regarded as an extremely safe, non invasive diagnostic technique. MRI may, however, pose a threat to patients that have implantable devices, such as, without limitation, a deep brain stimulation (DBS) device, a pacemaker, a neurostimulator, or a cardio defibrillator. Currently, patients with metallic implants are not allowed to undergo an MRI scan. One of the main reasons for this is the excessive heating caused by the electromagnetic field concentration around the leads of an implant during an MRI procedure.
Many cases with substantial temperature increase during MRI scanning have been reported and reviewed. For example, in Achenbach S, Moshage W, Diem B, Bieberle T, Schibgilla V, Bachmann K., “Effects of Magnetic Resonance Imaging on Cardiac Pacemakers and Electrodes,” Am Heart J 1997; 134:467-473, a maximum temperature increase of 63.1° C. was reported during 90 seconds of MRI scanning. Additionally, in an in vitro evaluation of 44 commercially available pacemaker leads, it was reported in Sommer T, Hahlhaus C, Lauck G, et al., “MR Imaging and Cardiac Pacemakers: In Vitro Evaluation and In Vivo Studies in 51 patients at 0.5 T.,” Radiology 2000; 215:869-879, that a temperature increase of 23.5° C. was observed in a 0.5 Tesla experiment. Substantial temperature increases were also observed in MRI scans involving patients with neurostimulators, as reported in Gleason C A, Kaula N F, Hricak H, et al., “The Effect of Magnetic Resonance Imagers on Neurostimulators,” Pacing Clin Electrophysiology 1992; 15:81-94. Furthermore, 1.5 T and a SAR of 3.0 W/kg have been shown to cause severe necrosis in the mucous membranes of dogs with transesophageal cardiac pacing leads as reported in Hofman M B, de Cock C C, van der Linden J C, et al., “Transesophageal Cardiac Pacing During Magnetic Resonance Imaging: Feasibility And Safety Considerations,” Magn Reson Med 1996; 35:413-422.
Moreover, a 16.8° C. temperature increase on a half wavelength wire in a gel-phantom experiment was observed and reported in Smith C D, Kildishev A V, Nyenhuis J A, Foster K S, Bourland J D, “Interactions Of MRI Magnetic Fields With Elongated Medical Implants,” J Applied Physics 2000; 87:6188-6190. As reported in Konings M K, Bartels L W, Smits H J, Bakker C J, “Heating Around Intravascular Guidewires By Resonating RF Waves,” J Magn Reson Imaging 2000; 12:79-85, temperature increases due to endavascular guidewires between 26° C. and 74° C. were observed in saline bath experiments of up to 30 seconds of scan time. In another experiment with saline solution, reported in Nitz W R, Oppelt A, Renz W, Manke C, Lenhart M, Link J., “On The Heating Of Linear Conductive Structures As Guide Wires And Catheters In Interventional MRI,” J Magn Reson Imaging 2001; 13:105-114, up to 34° C. of temperature increase was observed for a half wavelength wire. It should be noted that first, second or third order bums were observed in many of the in-vivo studies mentioned above.
A recent study was performed for one of the most widely used neurostimulation systems, the Activa Tremor Control System sold by Medtronic. Different configurations were evaluated to assess worst case and clinically relevant positioning scenarios, and in vitro experiments were performed at 64 MHz MR system using gel phantoms to represent human tissue. As reported in Rezai A R, Finelli D, Nyenhuis J A, et al., “Neurostimulator For Deep Brain Stimulation: Ex Vivo Evaluation Of MRI-Related Heating At 1.5-Tesla,” J Magn Reson Imaging 2002; 15:241-250, the highest temperature change observed was 25.3° C. for the RF coil and 7.1° C. for the head coil. These results indicate that heating may be hazardous under certain MRI scanning conditions.
The FREEHAND System Implantable Functional Neurostimulator is a commercially available RF-powered motor control neuroprosthesis that consists of both implanted and external components sold by NeuroControl Corporation of Cleveland, Ohio. Findings from of an MRI-induced heating experiment during which the FREEHAND System was exposed to a whole-body-averaged SAR of 1.1 W/kg for 30 minutes showed that localized temperature increases were no greater than 2.7° C. with the device in a gel-filled phantom. A patient with a FREEHAND system can thus only undergo an MRI procedure under certain input power levels for a 1.5 Tesla scanner.
Due to the safety concerns created by the potential for excessive heating as described above, several strategies have been developed to promote MRI safety for patient's having metallic implants. One of the basic ones is to set a power threshold that ensures only a reasonable amount of heating will occur. A methodology for such a power limitation was previously published in Yeung C J, Susil R C, Atalar E., “RF Safety Of Wires In Interventional MRI: Using A Safety Index,” Magn Reson Med 2002; 47:187-193. However, many modem MRI pulse sequences, such as fast spin-echo or steady-state free precession (SSFP), require high RF power levels and therefore there is no guarantee that good quality images can be acquired if RF power is limited.
Most of the studies on the heating of metallic implants concentrate on the heating of the leads of the implant rather than the generator of the implant. This is primarily due to the fact that generators are typically smooth devices with curved edges and are therefore less threatening structures than the leads in terms of electromagnetic field concentration. As a result, less heating is observed and smaller temperature increase is expected in generators. See, for example, the results reported in Ferhanoglu O, Tasci O. T, El-Sharkawy A, Altintas A, Atalar E, “Investigating RF Heating Of Pacemakers In MRI Using A Safety Index”, Proc. International Society of Magnetic Resonance in Medicine, 12th Scientific Meeting, Kyoto, 2004, and Ferhanoglu O, El-Sharkawy A, Atalar E, “RF Heating At The Tip Of Pacemaker Leads,” Proc. European Society of Magnetic Resonance in Medicine and Biology, 21st Scientific Meeting, Copenhagen, 2004.
U.S. Pat. No. 6,284,971 discloses a coaxial cable which may be a magnetic resonance imaging coaxial cable designed for enhanced safety so as to reduce the risk of excessive heating or bums to a user. The cable has an elongated axially oriented inner conductor and an axially oriented outer shield conductor in spaced relationship with respect thereto with a first dielectric material disposed therebetween. However in this design, high permittivity materials must be employed. This requirement may create flexibility problems since high permittivity materials are brittle and rigid. In addition, there may be more than one the lead which may require usage of separate coaxial cables. In such a case, miniaturization of the design is a difficult task.
RF chokes and filters have been used in several previous studies. For example, as described in Susil R C, et al., “Multifunctional Interventional Devices for MRI: A Combined Electrophysiology/MRI Catheter”, MRM 47:594-600 (2002), RF chokes were used in the design of a combined electrophysiology/MRI catheter, and as described in Ladd M E, et. al., “Reduction of Resonant RF Heating in Intravascular Catheters Using Coaxial Chokes”, MRM 43:615-619 (2000), triaxial chokes were used to present a high impedance to currents flowing on the outer surface of the triax.
U.S. Pat. No. 6,539,253 discloses an implantable medical device incorporating integrated circuit notch filters, and U.S. Pat. No. 5,817,136 discloses a pacemaker with EMI protection. Both of the designs ensure electromagnetic interference is not a problem, however safety in terms of heating is not guaranteed. High current may still be flowing through long cables and these high currents may cause excessive heating and buns.
U.S. Pat. No. 5,217,010 describes optical signal transmission in between the generator and the organ in a pacemaker, which provides safety since there is no coupling with the optical system and the electromagnetic field. However, the electrical to optical and optical to electrical energy conversion efficiency is limited and therefore the lifetime of the pulse generator is reduced significantly. Miniaturization in this case is also a difficult task.
It is thus apparent that a need exists for an electrical lead which may be used with, for example, metallic implants, which resists the induction of currents from an external electromagnetic field, such as the field that is present during MRI scanning, and therefore reduces the likelihood of excessive heating from such fields.